Sodium polyphosphates have been used as the builder of choice in previous aqueous cleaning solutions, but because of the increased use of liquid detergents, where sodium tripolyphosphate has a limited solubility, and increased environmental concerns on the use of phosphorous containing builders, alternative compositions have been investigated. However, with the decrease in phosphate use, performance of the cleaners has also decreased.
Attempts have been made to replace sodium tripolyphosphate with sodium carbonate. Sodium carbonate has been found to effectively clean soil from dishes when combined with sodium silicate, which prevents corrosion of glass and metal during the wash. It has been found that the introduction of sodium silicate results in the formation of magnesium silicate, due to free magnesium which is present in household water supplies. The magnesium silicate forms thin films over substrates in the wash, the film being most readily visible on metal and glass. Over time, repeated deposition of magnesium silicate film on metal results in the development of a blue coloration of the metal substrates. In order to prevent coloration of metal dishware, and the metal components of automatic dishwashing machines, sodium silicate levels must be minimized in the liquid composition. However, some level of silicate must be present in order to prevent corrosion of glass and metal. Previous attempts to minimize the silicate level in nil-phosphate compositions have resulted in liquid compositions which exhibit poor viscosity and overall rheological profiles. It is well known that achieving suitable rheological parameters is important to signal to consumers that the composition provides sufficient cleaning of dishware.
Based on the foregoing, there is a need for a liquid automatic dish washing composition, which is substantially free of phosphate, and which minimizes visible filming of metal substrates during the wash, while maintaining desirable rheological attributes.